Individuals who suffer anterior cruciate ligament (ACL) injury and surgical reconstruction are at a substantially heightened risk of developing knee osteoarthritis (OA). Compared to the general population in whom knee OA typically develops later in life following decades of gradual joint degeneration, signs of knee OA have been identified within 1 year following ACL injury. However, the factors that contribute to knee OA development, and thus the targets for prevention efforts, are unclear.

Quadriceps dysfunction is a common lingering problem that patients face after ACL injury and reconstruction, and has been identified up to 20 years following injury. The quadriceps muscle plays a critical role in absorbing impact forces during tasks such as walking and running. When the quadriceps doesn’t work properly, its ability to absorb these impact forces is diminished, and cartilage in the joint experiences greater loading. Given the repetitive nature of walking (5,000+ steps per day), this greater loading potentially contributes to breakdown of cartilage in the knee joint and development of knee OA.

Knee OA results from gradual breakdown of cartilage within the joint. Cartilage is sensitive to the rate at which it is loaded, with higher loading rates resulting in greater cartilage damage compared to lower loading rates. Greater loading rates, known as impulsive loading, have been identified during walking individuals diagnosed with knee OA, but it is unclear if this characteristic is present in individuals with ACL injury and reconstruction.

The purposes of this study were to 1) determine if individuals who have experienced ACL injury and surgical reconstruction display impulsive loading and 2) evaluate the influence of quadriceps dysfunction on gait biomechanics linked to knee OA development.

What did you do and what did you find in this study?

We measured quadriceps function and walking gait biomechanics in individuals who had received ACL reconstruction surgery an average of 48 months prior to participation. Three-dimensional gait biomechanics were captured as subjects walked across force plates embedded in a walkway. The primary variables of interest were the loading rates of the overall peak vertical ground reaction force (c in the figure to the right) and the peak force immediately following heelstrike (a in the figure to the right), as these variables indicate how rapidly forces are applied to the limb. Several indices of quadriceps function were measured including isometric strength and rate of torque development, isokinetic strength and power, and the central activation ratio (i.e. quantification of an individual’s ability to voluntarily activate a muscle).

Loading rates were significantly greater in the ACL reconstructed limb than in the healthy contralateral limb, indicating that these individuals experience impulsive loading that likely contributes to development of knee OA. Additionally, greater quadriceps dysfunction, particularly a lower rate of torque development (i.e. how quickly the muscle can produce force), was associated with greater loading rates. These results support the notion that quadriceps dysfunction following ACL injury and reconstruction leads to aberrant gait biomechanics, and potentially to development of knee OA.

How do these findings impact the public?

The results of this study highlight the importance of enhancing quadriceps function during rehabilitation of ACL injuries. Current rehabilitation methods are typically insufficient for achieving these goals as evidenced by the chronic nature of quadriceps dysfunction following ACL injury. Future research is necessary to identify novel techniques for enhancing quadriceps function and reducing the risk of knee OA following ACL injury. The annual lifetime health care burden of ACL injury (i.e. the total cost of treatment per year over the average lifespan) is as high as $18 billion. Furthermore, knee OA in general affects 29 million Americans at an annual cost of $165 billion, and is a leading cause of disability. These results may provide essential information for developing interventions for reducing health care costs and disability.